Remark on the Farey fraction spin chain

In 1999, Kleban and \"Ozl\"uk introduced a `Farey fraction spin chain' and made a conjecture regarding its asymptotic number of states with given energy, the latter being given (up to some normalisation) by the number $\Phi(N)$ of $2\times2$ matrices arising as products of $\bigl(\!\begin{smallmatrix} 1 & 0 \\ 1 & 1 \end{smallmatrix}\!\bigr)$ and $\bigl(\!\begin{smallmatrix} 1 & 1 \\ 0 & 1 \end{smallmatrix}\!\bigr)$ whose trace equals $N$. Although their conjecture was disproved by Peter (2001), quite precise results are known on average by works of Kallies--\"Ozl\"uk--Peter--Snyder (2001), Boca (2007) and Ustinov (2013). We show that the problem of estimating $\Phi(N)$ can be reduced to a problem on divisors of quadratic polynomials which was already solved by Hooley (1958) in a special case and, quite recently, in full generality by Bykovski{\u{\i}} and Ustinov (2019). This produces an unconditional estimate for $\Phi(N)$, which hitherto was only (implicitly) known, conditionally on the availability on wide zero-free regions for certain Dirichlet $L$-functions, by the work of Kallies--\"Ozl\"uk--Peter--Snyder.